Radio-frequency power amplifiers are essential components of transmitters found in radio communication systems, and are deployed in various applications, such as mobile telephony, broadcast, wireless data networking, radiolocation and other fields. Generally, they function to make copies of their inputs, which are signals generated by other components of communication equipment, such as base transmitters, mobile devices, or the like, where the copies or output signals are powerful enough to propagate for appropriate distances. Two often conflicting requirements that constrain radio frequency power amplifiers are linearity and efficiency.
The linearity requirement or constraint on a radio frequency power amplifier is that it reproduce the form of its input signal faithfully. Small distortions in the form of the output signal relative to the input can cause the radio frequency power amplifier output signal to interfere with other radio services, in violation of regulatory requirements, or make it difficult or impossible to receive/demodulate the signal accurately. These distortions may be caused, for example, by the fact that the characteristics of the components of which a radio frequency power amplifier is composed (e.g. transistors) are non-ideal, e.g., vary with the electrical currents that they carry, which necessarily include the signal being reproduced. A conventional method (“class A operation”) of getting good linearity in this situation is to add a large “bias” current to signal currents so that current variations due to the signal are small in comparison.
The efficiency requirement or constraint means that the amplifier should not consume excessive power relative to its desired output power: thus, for example, an amplifier required to produce 10 Watts of output power may typically consume 100 Watts. This is often caused by the use of large bias currents, as described above, to improve linearity. The power (90 Watts in the example) “wasted” in this way causes many problems. For example, the power dissipated is manifested as heat, which has to be removed—often with large heat sinks and fans—before it causes temperature rises that damage the amplifier or other circuits. When equipment is battery-operated (e.g. in cell phones or in fixed installations (base transmitters) that are running on backup batteries during a power failure), battery size and hence weight and cost increases directly with power requirements.
Relatively efficient power amplifier circuits are known, and for radio frequency power amplifiers one of the more efficient is known as type or class “E”. These amplifiers attempt to operate their transistors as pure switches, which in principle dissipate (and hence waste) no power. Their operation depends on synchronization between closing the “switch” device and the “ringing” of a resonant load circuit, such that the switch is only driven closed at times when the voltage across it is almost zero. However, class E amplifiers pose problems. For example, since their output power is effectively set by a power supply voltage, they are difficult to amplitude-modulate and attempts to do so have resulted in both poor efficiency and poor linearity. The inability to modulate amplitude severely limits the applicability of class E amplifiers in most modern systems employing complex forms of modulation with varying amplitude or amplitude inverting signals.
Another switching power amplifier is known as class “D”. This amplifier architecture has been used for audio-frequency applications. Class D amplifiers in theory have low power dissipation (e.g. a switch does not dissipate power). In practice, Class D amplifiers are continually discharging capacitance (e.g., when turned on) and this can amount to significant power dissipation at radio frequencies.
Sigma delta technology is a known technique that allows feedback to be used to linearize, for example, class “D” switching amplifiers for audio-frequency use, but ordinarily this technology requires that switching events be synchronous to a fixed clock frequency. Typically, a sigma delta loop samples the output of a loop filter at a fixed rate that is independent of any input signal. This causes problems for many radio frequency power amplifiers since their inputs need to be synchronized with a high frequency signal. Note that sigma delta and delta sigma are expressions that may be used interchangeably in this document.